This invention relates to rare-earth-iron magnets coated on the surface for corrosion resistance. The invention further relates to rare-earth-iron magnets having a surface resistant to impacts as well as to corrosive attacks.
Rare-earth-iron magnets have recently attracted attention as new high-energy-product magnets because of their cost and machinability advantages over, and greater energy product than, samarium-cobalt magnets usually used for the purposes. Among the magnets of this type, a formulation consisting of 8 to 30 percent rare-earth element, 2 to 28 percent boron, and the balance iron and inevitable impurities, all in atomic ratio, has been found particularly effective.
However, the rare-earth-iron magnets are inferior in corrosion resistance to the Sm-Co system. To overcome this disadvantage, various surface treatments are being investigated. Inadequate impact resistance is another problem yet to be solved.
Rare-earth-iron magnets are produced by sintering or quenching. Magnets of this system contain much Nd and Fe both of which oxidize easily, and are susceptible to attacks by chemicals, especially acids and alkalis. Surface treatments, such as wet plating, tend to invite surface corrosion during pretreatment with acid or alkali or in the course of plating process. Even a magnet plated well can show reduced magnetic characteristics due to internal or intercrystalline corrosion under the influence of some chemical which has intruded. The materials made by quenching undergo less deterioration of magnetic characteristics with distortion by external forces or with heat than the materials obtained by sintering. However, quenched powders frequently are used with a plastic binder or the like, and high adhesion strength as coating materials is required for both the surface magnetic material and the binder material.
One solution known in the art to this problem is providing a plasma-polymerized film as a surface coating on the magnets (see Japanese Patent Application Public Disclosure No. 6811/1988). However, this process has a drawback in that the thicker the film the easier is the peeling of the protective coating film due to the internal stress developed in the protective film.
Another drawback of the plasma polymer film is that with the ordinary multi-element system a sufficient degree of polymerization can hardly be attained.
When acrylic acid or the like is used, for example, active oxygen is present while plasma polymerization is in progress. It thus causes plasma etching simultaneously with the plasma polymerization. Consequently, the resulting protective polymer film has insufficient hardness and density, and its degree of polymerization is low. Hence it provides an inadequate gas barrier. In addition, the presence of oxygen permits introduction of OH and other hydrophilic groups into the polymer film, rendering it difficult for the latter to function satisfactorily as an anticorrosive protective film.
Attempts have also been made to form a high-molecular-weight resin film as a protective coating on rare-earth sintered metal magnets (e.g. Japanese Patent Application Public Disclosure Nos. 198221/1986, 81908/1981, 63901/1985). High-molecular-weight resins cannot produce adequate bond to the metal surfaces, however, because they have high moisture and oxygen permeabilities and low affinity for the rare-earth sintered metal magnets, with the result that such films cannot provide satisfactory corrosion resistance. Among those resins, fluorocarbon resin and the like which require high-temperature baking can oxidize magnets, whereas epoxy resins and the like are inferior in anti-corrosion properties. Thus, no film has been provided which combines good adhesion with corrosion resistance.
The use of a high corrosion-resistant film, such as of polyxylylene, has been proposed, but the adhesion is extremely low (a vapor-phase polymerization process for the film was advocated by Union Carbide Corp. of the United States, and is commercially available). Forming a polyxylylene film by vacuum evaporation has also been introduced, but the resulting film has too low a degree of polymerization and its corrosion resistance is questionable (Japanese Patent Application Public Disclosure No. 103714/1980).
Thus, it has hitherto been impossible with conventional techniques to produce a rare-earth-iron magnet having a protective coating film which can adhere firmly to the magnet and exhibit satisfactory anticorrosive functions.
The present invention, therefore, aims at providing a rare-earth-iron magnet having a highly corrosion-resistant protective coating film of polyparaxylylene solidly adhering to the magnet and also providing a process for producing the same.